Semisubmerged or small water plane area twin hull ships, sometimes referred to as SWATH ships, have been developed for high-speed operation at high sea states. U.S. Pat. Nos. 3,623,444 and 3,897,744 issued to Thomas G. Lang, disclosed ships of this configuration which have better operational characteristics than conventional ships and can operate at much higher sea states.
The above noted patents point out a number of configurations for such vessels. All of the configurations noted in these patents include lower hulls that are parallel to the design water line. However, the movement of the swath vessel through the water, particularly at medium to high speed, causes the water surfaces to become depressed along the length of the hulls. This, in effect, causes the hulls to be misaligned with the flow so that the swath vessel is running in a bow-down condition. Such a trim condition decreases the ship's stability and increases the possibility of propeller broaching and ventilating.
To correct such a trim condition it is necessary to compensate with trim control surfaces or the like to balance the ship from its bow-down condition. This, in turn, requires the application of additional power to compensate for the drag caused by the trim control surfaces. Moreover, the trim of the vessel is inherently unstable and requires constant adjusting of the trim surfaces.